1. Field of the Invention
The present invention relates in general to an apparatus and method for detecting atmospheric pressure in an internal combustion engine, and more particularly to a technique for detecting atmospheric pressure by using an aspiration pressure sensor during operation of the internal combustion engine.
2. Description of the Related Art
In general, the aspiration pressure sensor detects aspiration pressure at the lower reaches of a throttle valve in the internal combustion engine. When an open angle of the throttle valve is at full opening position, the aspiration pressure detected by the aspiration pressure sensor is roughly equal to the atmospheric pressure.
A Japanese Patent Publication No. Heisei 7-35749, published on Apr. 19, 1995, exemplifies a previously proposed apparatus for detecting atmospheric pressure in an internal combustion engine.
In the disclosure of the above-mentioned Japanese Patent Publication, when the open angle of the throttle valve is at a full opening position, atmospheric pressure is detected by the aspiration pressure sensor. If atmospheric pressure is only detected when the open angle of the throttle valve is at the full opening position, it is difficult to have a sufficient opportunity to accurately detect the atmospheric pressure. Therefore, when the open angle of the throttle valve is above a threshold level, the pressure detected by the aspiration pressure sensor is assumed to be the atmospheric pressure.
However, in a case of detecting atmospheric pressure when the open angle of the throttle valve is above the threshold level, the most recent detected value of the aspiration pressure sensor is assumed to be the atmospheric pressure, which value always updates a previously-stored value. As a result, when the open angle of the throttle valve decreases below the threshold level after full opening of the throttle valve, the storage value is updated by a low precision value which is detected by the lower open angle of the throttle valve, and this value replaces the previously-stored value corresponding to a full opening of the throttle valve.
Though the detecting precision can be maintained by selecting the threshold level closer to the full opening of the throttle valve, it is still difficult to have a sufficient opportunity to accurately detect the atmospheric pressure.